(1) Field of the Invention
This invention relates to the cladding of metal ingots used for the formation of metal sheet by rolling. More particularly, the invention relates to cladding such ingots on one or both faces thereof.
(2) Description of the Related Art
It is well known to produce sheets of metal (particularly, although not exclusively, aluminum and aluminum alloys) provided with a core layer of one metal (e.g. AA3003 containing aluminum and 1% by weight Mn) and a thin cladding layer of a different metal (e.g. a high silicon-content alloy, such as Al plus 7 to 10% by weight Si) on one or both major surfaces. Such composite metal sheet is used, for example, as brazing sheet in which a core of a relatively high melting point aluminum alloy is clad with a thin layer of a low melting point alloy to permit attachment of metal components by brazing. In other cases, an alloy that is easily corroded may be clad with a corrosion-resistant alloy to prevent premature degradation.
Typically, clad metal sheet of this kind is produced by co-rolling of a core ingot and one or more plates of the cladding metal. Several hot-rolling and cold-rolling steps are normally undertaken to achieve the gauge reduction required and, during this process, the different metal layers become welded together by the heat and pressure of rolling. There is always a risk, however, that the plates or sheet layers will separate during the rolling process with potentially dangerous and damaging results. The likelihood of this is greatest at the start of rolling and, to prevent this, the cladding plate (or plates) may be secured to the core ingot by means of steel bands during a preheating step, in order to achieve a degree of bonding, followed by removal of the bands prior to rolling. This is a very demanding operation for the operators of the process given that the ingots may have a temperature in the region of 400° C. or more when the bands have to be removed.
Alternatively, the ingot and plate(s) may be welded together at their edges prior to rolling, but welding is time-consuming and is difficult or impossible at times due to warping of the plates.
U.S. Pat. No. 543,192, which issued to A. Rodig on Jul. 23, 1895 discloses a cladding process in which the cladding plates are physically interlocked to the core ingot by means of inter-engaging undercut or overhanging portions. The ingot is provided with furrows, grooves, recesses or overhanging projections and the cladding plate is formed with correspondingly shaped parts or is cast on the ingot surface.
U.S. Pat. No. 6,251,527 which issued to Schelin et al. (Alcoa) on Jun. 26, 2001 grooves the core ingot to provide a liner bed for receiving the cladding plate. A continuous upstanding lip provided at the edges of the ingot holds the cladding plate against slippage during rolling.
U.S. Pat. No. 6,818,078 which issued to Kim et al. (Liquidmetal Technologies) on Nov. 16, 2004 relates to a method of joining an amorphous metal to a non-amorphous metal. This is achieved by forming a cast mechanical joint between the two materials to create mechanical interlocking.
A problem encountered with mechanical interlocking or inlaying of metals of different kinds is that, during casting or cooling, thermal contraction of the metal or metals may result in cracking if one metal is mechanically fixed to another metal.